You can purge unused components in the Components panel when the In Model components are shown.
There’s a similar thing for Materials and Tags. In the desktop version there’s a way to do it for everything at once.
Probably. There really is no such thing as layers in SketchUp. There are tags which are used for visibility control.
That’s 2D application talk. SketchUp is a 3D modeling tool and as such Send to Front/Send to Back makes no sense. You’re working in 3D space so if you want one object to appear behind another, you need to actually move it back in space.
Again, no layers in SketchUp. Objects (groups and components) can get tags assigned to them to allow you to control the visibility of those objects. The order of the tags makes no difference for that like the order of layers does in 2D programs.
Key thing with SketchUp is to leave Untagged as active. ALL edges and faces remain untagged and only components and groups get other tags assigned to them.
What is your use for the model when you get it done? Are you only planning to make a 2D drawing or are you going to take this to 3D?